let bygones be bygones

verb1

1. •
   To disregard or ignore a past difficulty in a relationship or an offence (when dealing with another individual).

   • "Never mind, I'll give you a hundred dollars for it," said the king; "you did me out of horse and saddle the other day, and the bridle too, but I'll let bygones be bygones, if I get the pot." — 1881